Seismic and geodetic evidence for the existence of hot materials beneath the Wakayama swarm activity, southwestern Japan

Abstract

A remarkable seismic swarm area exists amid the Wakayama Plain on the western coast of the Kii Peninsula, in southwestern Japan, close to the Median Tectonic Line (MTL). The hypocenters of these swarm earthquakes are very shallow with a median of 6 km. This is exceptionally shallow for seismic activity in plain areas where the seismogenic layer is usually deep compared to that in mountainous regions ( Takayama et al., 2006). Also, the b-value below 8 km in the eastern part of this swarm region is anomalously high. In addition to these seismic features, GEONET data and leveling survey data obtained by the Geospatial Information Authority of Japan indicate that land around the swarm region are expanded and upheaved. Areas of the dilatation and upheaval almost overlap and their centers correspond to the site where the high b-value anomaly is observed. These facts strongly suggest that there are hot materials beneath the source region. In the Kii Peninsula acid volcanic rocks outcrop widely, and the Wakayama swarm area is located on the southern side of the MTL in the metamorphic belt below which the relatively young hot Philippine Sea plate is subducting and dehydration from a surpentinized forearc mantle is expected to occur. Based on the seismic and geodetic observations and the tectonic background of the area in which the swarm region is situated, we propose a hypothesis that a diorite or a serpentine diapir with plentiful hot fluids might be intruding beneath the source area.